Sensor And Method For Making The Same

ABSTRACT

A method of making a sensor comprises substantially laterally growing at least one nanowire having at least two segments between two electrodes, whereby a junction or connection is formed between the at least two segments; and establishing a sensing material adjacent to the junction or connection, and adjacent to at least a portion of each of the at least two segments, wherein the sensing material has at least two states.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional application of application Ser.No. 11/583,648, filed Oct. 19, 2006, the contents of which are herebyincorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made in the course of research partially supported bythe Defense Advanced Research Projects Agency, Agreement No.HR0011-05-3-0001. The U.S. government has certain rights in theinvention.

BACKGROUND

The present disclosure relates generally to sensors and methods formaking the same.

Since the inception of semiconductor technology, a consistent trend hasbeen toward the development of smaller device dimensions and higherdevice densities. As a result, nanotechnology has seen explosive growthand generated considerable interest. Nanotechnology is centered on thefabrication and application of nano-scale structures, or structureshaving dimensions that are often 5 to 100 times smaller thanconventional semiconductor structures. Nanowires are included in thecategory of nano-scale structures.

Nanowires are wire-like structures having diameters on the order ofabout 1 nm to about 100 nm. Nanowires are suitable for use in a varietyof applications, including functioning as conventional wires forinterconnection applications, as semiconductor devices, and as sensors.While holding much promise, the practical application of nanowires hasbeen somewhat limited. In particular, non-suspended nanowire sensingdevices tend to leak heat, and, in some instances, are incapable ofsensing relatively small temperature changes.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of embodiment(s) of the present disclosure willbecome apparent by reference to the following detailed description anddrawings, in which like reference numerals correspond to similar, thoughnot necessarily identical components. For the sake of brevity, referencenumerals or features having a previously described function may notnecessarily be described in connection with other drawings in which theyappear.

FIG. 1 is a schematic view of an embodiment of a sensor; and

FIG. 2 is a schematic view of another embodiment of a sensor.

DETAILED DESCRIPTION

Providing nanowires, especially nanowires with properties that can varyin response to chemical reactions, that can be integrated on a siliconplatform, and that can be fabricated in production quantities for areasonable cost has proven difficult.

Embodiment(s) of the sensor disclosed herein overcome one or more of thedrawbacks discussed hereinabove, as they are advantageously capable ofsensing the presence and/or quantity of a reaction (e.g., a chemicalreaction). The sensors incorporate nanowire(s) grown laterally betweentwo electrodes. The nanowires may advantageously have multiple segmentsof different conductivity types or different materials (e.g., differentfrom other segments and/or different from the electrode materials). Theconnection between the various segments of the nanowire mayadvantageously be electrically useful (e.g., an ohmic connection, ajunction (i.e., a diode), or the like).

A reaction occurring near the sensing material located at the connectionbetween the various nanowire segments is capable of generating ameasurable property. As used herein, the term “sensing material” refersto a material that has two discrete states (e.g., of the property to bemeasured), or a material that has more than two states, including acontinuum of states (i.e., the material is continuously variable). Theproperty may advantageously be used to determine the presence and/orquantity of the reaction, the concentration of the reaction reactants,or combinations thereof. Further, without being bound to any theory, itis believed that the species of the reaction reactants may be determinedby selecting a coating that preferentially and/or selectively bindsspecific reactants or reaction products.

Using a nanowire to sense chemical reaction(s) may be advantageous, inpart, because of the low thermal mass of the nanowire. It is believedthat the low thermal mass increases the sensitivity to small amounts ofreaction or small changes in the reaction, and decreases the responsetime for sensing the reaction or changes in the reaction. Furthermore,with suitable bias, both endothermic and exothermic reactions may besensed.

Referring now to FIG. 1, an embodiment of the sensor 10 includes twoelectrodes 12, 14 positioned on a substrate 16. In an embodiment, theelectrodes 12, 14 are formed from a layer of silicon (Si) cut orpolished with the surface plane being a (110) crystal lattice plane.Such a layer is referred to as a (110) oriented Si layer. As usedherein, the (110) plane is considered to be horizontally oriented withrespect to the Cartesian coordinate system. The (110) oriented layerfrom which the electrodes 12, 14 are formed further has (111) planes ofthe Si crystal lattice, at least some of which are approximatelyperpendicular to, and intersect with the horizontally oriented (110)surface. These intersecting (111) planes are referred to herein asvertically oriented (111) planes or surfaces, noting that the (111)planes are approximately vertically oriented relative to the horizontal(110) surface of the layer from which the electrodes 12, 14 are formed.The (110) Si layer may be etched anisotropically using an etchant, suchas KOH. The (111) planes etch slowly compared to other crystal planes;as such, the resulting structure, which forms the electrodes 12, 14, isbounded by vertical (111) planes. These (111) planes form the sides ofthe electrodes 12, 14.

As used herein, the term “horizontal” generally refers to a direction ora plane that is parallel with a surface of the substrate 16 or wafer,while the term “vertical” generally refers to a direction or plane thatis substantially or approximately perpendicular to the substratesurface. It is to be understood that the specific use of the terms“horizontal” and “vertical” to describe relative characteristics is tofacilitate discussion and is not intended to limit embodiments of thepresent disclosure.

The electrodes 12, 14 may have first and second conductivity types. Itis to be understood that the first and second conductivity types may bethe same or different. In a non-limitative example, the firstconductivity type is p-type conductivity, and the second conductivitytype is n-type conductivity, or vice versa. In other embodiments, boththe first and second conductivity types are p-type conductivity orn-type conductivity.

If the property to be measured is an electrical property, the electrodes12, 14, are electrically isolated from each other except through ananowire 18. If the substrate 16 is formed of an insulating material,the electrodes 12, 14, are electrically isolated from each other. If thesubstrate 16 is formed of a conductive or a semi-conductive material, aninsulating layer 19 is established between the respective electrodes 12,14, and the substrate 16 to electrically isolate the electrodes 12, 14from each other.

The nanowire 18 is grown substantially laterally between the electrodes12, 14. In an embodiment, growth of the nanowire 18 is initiated at oneof the electrodes 12, 14, and a connection is formed at the other of theelectrodes 14, 12. It is to be understood, however, that the nanowire 18may be formed via any suitable method. A non-limitative example offorming a lateral nanowire 18 is described in U.S. patent applicationSer. No. 10/738,176, filed on Dec. 17, 2003 (U.S. Publication No.2005/0133476 A1, published Jun. 23, 2005), which is incorporated hereinby reference in its entirety. Other example methods for forming lateralnanowires 18 are described in “Ultrahigh-density silicon nanobridgesformed between two vertical silicon surfaces” by Islam et al., publishedin 2004 in volume 15 of Nanotechnology at pages L5-L8; and “A novelinterconnection technique for manufacturing nanowire devices” by Islamet al., published in 2005 in volume 80 of Appl. Phys. A at pages1133-1140.

As depicted, the nanowire 18 has at least two segments 20, 22. As thenanowire 18 grows, it may be doped with a dopant that is capable ofintroducing the first conductivity type or the second conductivity typeto one or more of the nanowire segments 20, 22. Dopants for introducingp-type conductivity into group IV semi-conductors include, but are notlimited to boron, other like elements, or combinations thereof; anddopants for introducing n-type conductivity into group IVsemi-conductors include, but are not limited to phosphorus, arsenic,antimony, other like elements, or combinations thereof. Differentdopants may be suitable for group III-V materials, such as, for examplegallium arsenide.

In the embodiment shown in FIG. 1, one of the segments 20, 22 has thefirst conductivity type, and the other of the segments 22, 20 has thesecond conductivity type. Generally, the first and second conductivitytypes are different so that a junction (e.g., a p-n junction) is formedbetween the two segments 20, 22. In a non-limitative example, the firstconductivity type is p-type conductivity, and the second conductivitytype is n-type conductivity, or vice versa. If the semiconductormaterial forming both segments 20, 22 of the nanowire is the samematerial, the junction is a p-n homojunction.

In another embodiment, one of the segments 20, 22 is formed of a firstmaterial, and the other of the segments 22, 20 is formed of a secondmaterial that is different than the first material, so that aheterojunction is formed between the two segments 20, 22. The materialsfor the segments 20, 22 may be selected to be of opposite conductivitytypes so that a p-n heterojunction is formed, or they may be of the sameconductivity type so that an isotype heterojunction is formed.

As a non-limiting example of the sensor 10 shown in FIG. 1, the nanowire18 and electrodes 12, 14 may form a first conductivity type-firstconductivity type-second conductivity type-second conductivity typestructure, where the nanowire 18 has a second conductivity type segment(e.g., segment 22 adjacent a second conductivity type electrode 12) anda first conductivity type segment (e.g., segment 20 adjacent a firstconductivity type electrode 14). A non-limitative example of such astructure is a p-type-p-type-n-type-n-type (p-p-n-n) structure, whichhas a p-p junction (at the electrode 14-segment 20 interface), a p-njunction (at the segment 20-segment 22 interface), and an n-n junction(at the segment 22-electrode 12 interface). The first conductivity typesegment 20 of the nanowire 18 is grown from the electrode 14 of thefirst conductivity type. During growth of the nanowire 18, growth of thefirst conductivity type segment 20 may be stopped, and a secondconductivity type segment 22 may be grown from the first conductivitytype segment 20. The conductivity type may be changed by changing thedopant-containing species reaching the region where semiconductormaterial is being added to continue growth of the nanowire 18.Alternatively, the dopant can be added after growth of the nanowire 18is complete. In the embodiment shown in FIG. 1, the second conductivitytype segment 22 forms a connection with the electrode 12 of the secondconductivity type. It is to be understood that the conductivity of therespective electrodes 12, 14 and segments 20, 22 may be altered asdesired.

In the embodiment shown in FIG. 1, a junction 24 is formed at theinterface of the nanowire segments 20, 22. As previously described, thejunction 24 is often a homojunction. In some instances, though, thejunction 24 is a heterojunction. In an embodiment, a sensing material 26is positioned adjacent to the junction 24 and adjacent to at least aportion of each of the nanowire segments 20, 22. It is to be understoodthat the sensing material 26 may also be positioned adjacent to otherportions of the nanowire segments 20, 22. The sensing material 26 may beapplied to the nanowire 18 via vapor deposition techniques, liquiddeposition techniques (including self-assembling of a monolayer), inkjetdeposition techniques, or the like, or combinations thereof. It is to beunderstood that the sensing material 26 may be positioned on, under,and/or so it substantially surrounds the junction 24 and the segment 20,22 portions.

As previously described, the sensing material 26 may have two discretestates (between which the material 26 is switchable), or it may havemore than two states, including a continuum of states. The two discretestates may be of the property to be measured, for example, a highresistance state and a low resistance state. The two discrete states mayrepresent “ON” and “OFF” states. In this embodiment, upon exposure to acertain reaction property, the sensing material 26 turns from ON to OFFor from OFF to ON. Generally, sensing materials 26 having two discretestates are suitable for measuring the presence or absence of a reaction,and sensing materials 26 with more than two states or a continuum ofstates are suitable for measuring an amount of the reaction.

The sensing material 26 may be a material that is switchable between atleast a high and a low resistance state, a plurality of nanoparticlescoated with spacer ligands, or combinations thereof. As a non-limitingexample, the sensing material 26 may be a material that has a highresistance state and a low resistance state, and is switchable from onestate to the other state when exposed to conditions that induceresistance changes (e.g., the reaction being measured). As anothernon-limiting example, the sensing material 26 may be a material with acontinuum of states so that its resistance is a continuous function oftemperature. In this case, the resistance measured indicates thetemperature of the material and, therefore, the intensity of thereaction at any instant of time. By integrating the changes over time,the quantity of the reaction that has occurred can be measured.

The embodiment shown in FIG. 1 depicts the plurality of nanoparticlescoated with space ligands as the sensing material 26. A reaction (e.g.,chemical or otherwise) may be generated at or near the junction 24 ofthe sensor 10. In an embodiment, heat is generated by the reaction. In anon-lighting example, the nanoparticles shown in FIG. 1 expand uponexposure to the generated heat, causing the spacing between thenanoparticles to decrease. This decreased space lowers the resistance ofa path in parallel with the junction 24.

In a non-limiting example, the nanoparticles may act as a binary sensingmaterial that undergoes a Mott transition (switches from a nonconductiveor “OFF” state to a conductive or “ON” state) when exposed to heat thatis generated, for example, from such a reaction (e.g., chemical orotherwise). As such, the heat and/or the change in resistance isindicative of the chemical reaction occurring near the junction 24. Inthis embodiment, the resistance may be measured, and such measurementsmay be used to determine the presence of the reaction, the concentrationof the reaction reactants, or combinations thereof.

Very generally, the change of resistance of the sensing material 26shown in FIG. 1 depends, at least in part, on the temperature, andconsequently on the amount of heat generated by the reaction. The amountof heat generated depends, at least in part, on the quantity of thereaction, and therefore on the concentration of the reactants. If thechange is abrupt, the presence of a reaction is detectable. If however,the change is gradual with temperature, the reaction presence and thereactants' concentration are detectable.

Embodiments of the sensor 10 having a junction may be biased so thecurrent flowing through the nanowire 18 is less than the current carriedby the sensing material 26 in its conductive state. The junction may bereverse biased, or biased with a low forward bias so that the currentflow through the junction 24 is low. A change in the parallelconductance through the sensing material 26 is then readily measuredbetween the electrodes 12, 14.

Referring now to FIG. 2, another embodiment of a sensor is generallydepicted at 10′. The substrate 16 and electrodes 12, 14 described inreference to FIG. 1 are suitable for use in the embodiment of the sensor10′ shown in FIG. 2.

In this embodiment, the nanowire 18 is grown substantially laterallybetween the electrodes 12, 14, and includes at least two segments 20′,22′ having a connection 28 therebetween. It is to be understood that thenanowire 18 is formed via embodiments of the methods disclosedhereinabove. Generally, the nanowire segments 20′, 22′ in thisembodiment are selected from metals, semi-conductor materials, orcombinations thereof. Non-limiting examples of such materials includesilicon, germanium, indium phosphide, gallium arsenide, boron, gold, orthe like, or combinations thereof.

The nanowire 18 depicted in FIG. 2 also includes an insulating material30 located at the connection 28 between the segments 20′, 22′. Theinsulating material 30 may be grown as a segment of the nanowire 18 bychanging the gaseous species to which the catalyzing nanoparticle (whichforms the nanowire 18) is exposed. Non-limiting examples of theinsulating material 30 include gallium aluminum arsenide, or the like.

In the embodiment shown in FIG. 2, the sensing material 26 is positionedadjacent to: the connection 28; the insulating material 30; and at leasta portion of each of the segments 20′, 22′. It is to be understood thatthe sensing material 26 may also be positioned adjacent to otherportions of the nanowire segments 20′, 22′. The sensing material 26 maybe established via any of the techniques described hereinabove. It is tobe understood that the sensing material 26 may be positioned on, under,and/or so it substantially surrounds the connection 28, the insulatingmaterial 30, and the portions of segments 20′, 22′.

As previously described, the sensing material 26 may be a resistivematerial that is changeable between at least a high and a low resistancestate, a plurality of nanoparticles coated with spacer ligands, orcombinations thereof. The embodiment shown in FIG. 2 depicts theresistive material that is switchable between the high and lowresistance states.

Similar to the embodiment of the sensor 10 shown in FIG. 1, a reaction(e.g., chemical or otherwise) may be generated at or near the connection28 of the sensor 10′. The resistance of sensing material 26 changes to adifferent state upon exposure to at least some measurable property(e.g., heat, temperature increase or decrease, etc.) generated by thereaction. This exposure results in a change in the resistance of a pathin parallel with the connection 28. Depending, at least in part, on theproperty and amount generated during the reaction, the resistance may beincreased or decreased. Furthermore, the exposure to the measurableproperty may cause the material 26 to switch from one resistance stateto another (generally suitable for detecting presence of a reaction), orto move through more than two states or a continuum of states ofdifferent resistance (generally suitable for detecting reaction presenceand the reactants' concentration).

For an exothermic reaction, the resistance of the sensing material 26shown in FIG. 2 usually decreases to a lower state upon exposure to atleast some measurable property generated by the reaction. This resultsin a decrease in the resistance of a path in parallel with theconnection 28.

The change in resistance is indicative of the chemical reactionoccurring near the connection 28. The resistance may be measured, andsuch measurements may be used to determine the presence and/or quantityof the reaction, the concentration of the reaction reactants, thespecies of the reaction reactants, or combinations thereof.

For the embodiments of the sensor 10, 10′ disclosed herein, the nanowire18 may include a junction 24 between differently doped segments 20, 22,of the same material, a junction 24 between segments 20, 22, ofdifferent materials, or an insulating material 30 between two conductingsegments 20′, 22′. The sensing material 26 may include materials thatare capable of switching between two states, or moving between more thantwo states or through a continuum of states. The sensors 10, 10′ shownin FIGS. 1 and 2 are non-limiting examples, and it is to be understoodthat different combinations of the nanowire 18 and sensing material 26are considered to be within the purview of the present disclosure.

While several embodiments have been described in detail, it will beapparent to those skilled in the art that the disclosed embodiments maybe modified. Therefore, the foregoing description is to be consideredexemplary rather than limiting.

1. A method of making a sensor, comprising: substantially laterally growing at least one nanowire having at least two segments between two electrodes, whereby a junction or connection is formed between the at least two segments; and establishing a sensing material adjacent to the junction or connection, and adjacent to at least a portion of each of the at least two segments, wherein the sensing material has at least two states.
 2. The method as defined in claim 1 wherein the at least two segments are formed of a conductive material, and wherein the method further comprises establishing an insulating material between the at least two segments at the connection.
 3. The method as defined in claim 2, further comprising establishing the sensing material adjacent to the insulating material and at least a portion of each of the at least two segments.
 4. The method as defined in claim 1, further comprising: doping a portion of the nanowire with a dopant configured to introduce a first conductivity type into one of the at least two segments; and doping an other portion of the nanowire with a dopant configured to introduce a second conductivity type into an other of the at least two segments.
 5. The method as defined in claim 4 wherein the first conductivity type is one of a p-type and an n-type, and wherein the second conductivity type is an other of the n-type and the p-type.
 6. The method as defined in claim 1 wherein the sensing material is selected from a material having a resistance changeable between at least a high resistance state and a low resistance state, a plurality of nanoparticles coated with spacer ligands, and combinations thereof. 